- UNDERSTANDING CAMERA LENSES -

Understanding camera lenses can help add more creative control to digital
photography. Choosing the right lens for the task can become a complex trade-off
between cost, size, weight, lens speed and image quality. This tutorial aims
to improve understanding by providing an introductory overview of concepts relating
to image quality, focal length, perspective, prime vs. zoom lenses and aperture
or f-number.

LENS ELEMENTS & IMAGE QUALITY

All but the simplest cameras contain lenses which are actually comprised
of several "lens elements." Each of these elements aims to direct the path of
light rays such that they recreate the image as accurately as possible on the
digital sensor. The goal is to minimize aberrations, while still utilizing the
fewest and least expensive elements.

Optical aberrations occur when points of the image do not translate back
onto single points after passing through the lens, causing image blurring, reduced
contrast or misalignment of colors (chromatic aberration). Lenses may also suffer
from uneven, radially decreasing image brightness (vignetting) or distortion.
Try moving your mouse over each of the options below to see how these can impact
image quality for extreme cases.

Original Image

Loss of Contrast

Blurring

Chromatic Aberration

Distortion

Vignetting

Original

Any of the above problems is present to some degree with any lens.
In the rest of this tutorial, when a lens is referred to as having lower
optical quality than another lens, this is manifested as some combination of
the above artifacts. Some of these lens artifacts may not be as objectionable
as others, depending on the subject matter.

INFLUENCE OF LENS FOCAL LENGTH

The focal length of a lens determines its angle of view, and thus also how
much the subject will be magnified for a given photographic position.
Wide angle
lenses have small focal lengths, while
telephoto lenses
have larger corresponding focal lengths.

Note: The location where light rays cross is not necessarily
equal to the focal length, as shown above, but is instead roughly proportional
to this distance. Therefore longer focal lengths still result in narrower
angles of view, as depicted.

Required Focal Length Calculator

Subject Distance:

Subject Size:

Camera Type:

Approximate
Required Focal Length:

Note: Calculator
assumes that camera is oriented such that the maximum
subject dimension given by "subject size" is in the camera's
longest dimension. Calculator not intended for use in
extreme macro photography, but does take into account small
changes in the angle of view due to focusing distance.

A more advanced focal length calculator
is shown here below. This page is useful to calculate the required lens
focal length to take a picture of an object of size yo at a distance
xo in order to generate an image of size yi on the film (or the CCD/C-MOS
imager). The film plane is located at a distance xi.

;

;

;

Distances xo and xi are measured from the lens front and rear
principal points respectively. Too often, unfortunately, these
points are not specified by the lens manufacturer: it's hard to
guess their position, since they can be anywhere inside or outside
the lens. If you use a commercial camera with a compatible lens on
it the distance xi is already adjusted to allow a perfect focus and
you don't have to worry. Anyway, the rear principal point is always
one focal length ahead the film plane when the lens is focused at
infinity.

Object size:

yo =

Image size:

yi =

Object distance:

xo =

Magnification:

M =

Focal length:

f =

mm

Angle of view:

w =

°

Bellows factor:

B =

Many will say that focal length also determines the perspective of an image,
but strictly speaking, perspective only changes with one's location relative
to their subject. If one tries to achieve the same subjects filling the
frame with both a wide angle and telephoto lens, then perspective does indeed
change because one is forced to move closer or further from their subject.
For these scenarios only, the wide angle lens exaggerates or stretches perspective,
whereas the telephoto lens compresses or flattens perspective.

Perspective control can be a powerful compositional tool in photography,
and often determines one's choice in focal length (when one can photograph
from any position). Move your mouse over the above image to view an
exaggerated perspective due to a wider angle lens. Note how the subjects
within the frame remain nearly identical-- therefore requiring a closer position
for the wider angle lens. The relative sizes of objects change such that
the distant doorway becomes smaller relative to the nearby lamps.

The following table provides a overview of what focal lengths are required
to be considered a wide angle or telephoto lens, in addition to their typical
uses. Please note that focal lengths listed are just rough ranges,
and actual uses may vary considerably; many use telephoto lenses in distant
landscapes to compress perspective, for example.

Lens Focal Length*

Terminology

Typical Photography

Less than 21 mm

Extreme Wide Angle

Architecture

21-35 mm

Wide Angle

Landscape

35-70 mm

Normal

Street & Documentary

70-135 mm

Medium Telephoto

Portraiture

135-300+ mm

Telephoto

Sports, Bird & Wildlife

*Note: Lens focal lengths are for 35 mm equivalent cameras.
If you have a compact or digital SLR camera, then you likely have a different
sensor size. To adjust the above numbers for your camera, please use the
focal length converter in the
tutorial on digital camera sensor sizes.

Other factors may also be influenced by lens focal length. Telephoto
lenses are more susceptible to camera shake since small hand movements become
magnified within the image, similar to the shakiness experience while trying
to look through binoculars with a large zoom. Wide angle lenses are generally
more resistant to flare, partially because the designers assume that the sun
is more likely to be within the frame for a wider angle of view. A final
consideration is that medium and telephoto lenses generally yield better optical
quality for similar price ranges.

FOCAL LENGTH & HANDHELD PHOTOS

The focal length of a lens may also have a significant impact on how easy
it is to achieve a sharp handheld photograph. Longer focal lengths
require shorter exposure times to minimize burring caused by shaky hands.
Think of this as if one were trying to hold a laser pointer steady; when shining
this pointer at a nearby object its bright spot ordinarily jumps around less
than for objects further away.

This is primarily because slight rotational vibrations are magnified greatly
with distance, whereas if only up and down or side to side vibrations were present,
the laser's bright spot would not change with distance.

A common rule of thumb for estimating how fast the exposure needs to be for
a given focal length is the one over focal length rule. This states
that for a 35 mm camera, the exposure time needs to be at least as fast as one
over the focal length in seconds. In other words, when using a 200 mm
focal length on a 35 mm camera, the exposure time needs to be at least 1/200
seconds-- otherwise blurring may be hard to avoid. Keep in mind that this
rule is just for rough guidance; some may be able to hand hold a shot for much
longer or shorter times than this rule estimates. For users of
digital cameras with cropped sensors, one needs to convert into a 35 mm
equivalent focal length.

ZOOM LENSES vs. PRIME LENSES

A zoom lens is one where the photographer can vary the focal length within
a pre-defined range, whereas this cannot be changed with a "prime" or fixed
focal length lens. The primary advantage of a zoom lens is that it is
easier to achieve a variety of compositions or perspectives (since lens changes
are not necessary). This advantage is often critical for dynamic subject
matter, such as in photojournalism and children's photography.

Keep in mind that using a zoom lens does not necessarily mean that one
no longer has to change their position; zooms just increase flexibility.
In the example below, the original position is shown along with two alternatives
using a zoom lens. If a prime lens were used, then a change of composition
would not have been possible without cropping the image (if a tighter composition
were desirable). Similar to the example in the previous section, the change
of perspective was achieved by zooming out and getting closer to the subject.
Alternatively, to achieve the opposite perspective effect, one could have zoomed
in and gotten further from the subject.

Two Options Available with a Zoom Lens:

Change of Composition

Change of Perspective

Why would one intentionally restrict their options by using a prime lens?
Prime lenses existed long before zoom lenses were available, and still offer
many advantages over their more modern counterparts. When zoom lenses
first arrived on the market, one often had to be willing to sacrifice a significant
amount of optical quality. However, more modern high-end zoom lenses generally
do not produce noticeably lower image quality, unless scrutinized by the trained
eye (or in a very large print).

The primary advantages of prime lenses are in cost, weight and speed.
An inexpensive prime lens can generally provide as good (or better) image
quality as a high-end zoom lens. Additionally, if only a small fraction
of the focal length range is necessary for a zoom lens, then a prime lens with
a similar focal length will be significantly smaller and lighter. Finally,
the best prime lenses almost always offer better light-gathering ability (larger
maximum aperture) than the fastest zoom lenses-- often critical for low-light
sports/theater photography, and when a
shallow depth of field is necessary.

INFLUENCE OF LENS APERTURE OR F-NUMBER

The aperture range of a lens refers to the amount that the lens can open
up or close down to let in more or less light, respectively. Apertures
are listed in terms of f-numbers, which quantitatively describe relative light-gathering
area (depicted below).

Note: Above comparison is qualitative; aperture opening (iris)
is rarely a perfect circle,
due to the presence of 5-8 blade-like lens diaphragms.

Note that larger aperture openings are defined to have lower f-numbers (often
very confusing). These two terms are often mistakenly interchanged; the
rest of this tutorial refers to lenses in terms of their aperture size.
Lenses with larger apertures are also described as being "faster," because
for a given ISO speed, the shutter speed can be made faster for the same exposure.
Additionally, a smaller aperture means that objects can be in focus over a wider
range of distance, a concept also termed the
depth of field.

f-#

Corresponding Impact on Other Properties:

Light-Gathering Area
(Aperture Size)

Required Shutter Speed

Depth of Field

Higher

Smaller

Slower

Wider

Lower

Larger

Faster

Narrower

When one is considering purchasing a lens, specifications ordinarily list
the maximum (and maybe minimum) available apertures. Lenses with a greater
range of aperture settings provide greater artistic flexibility, in terms of
both exposure options and depth of field. The maximum aperture is perhaps
the most important lens aperture specification, which is often listed on the
box along with focal length(s).

An f-number of X may also be displayed as 1:X (instead of f/X), as shown
below for the Canon 70-200 f/2.8 lens (whose box is also shown above and lists
f/2.8).

Portrait and indoor sports/theater photography often requires lenses with
very large maximum apertures, in order to be capable of faster shutter speeds
or narrower depth of fields, respectively. The narrow depth of field in
a portrait helps isolate the subject from their background. For digital
SLR cameras, lenses with larger maximum apertures provide significantly brighter
viewfinder images--
possibly critical for night and low-light photography. These also
often give faster and more accurate auto-focusing in low-light.
Manual focusing is also easier because the image in the viewfinder has
a narrower depth of field (thus making it more visible when objects come into
or out of focus).

Typical Maximum Apertures

Relative Light-Gathering Ability

Typical Lens Types

f/1.0

32X

Fastest Available Prime Lenses
(for Consumer Use)

f/1.4

16X

Fast Prime Lenses

f/2.0

8X

f/2.8

4X

Fastest Zoom Lenses
(for Constant Aperture)

f/4.0

2X

Light Weight Zoom Lenses or Extreme Telephoto Primes

f/5.6

1X

Minimum apertures for lenses are generally nowhere near as important as maximum
apertures. This is primarily because the minimum apertures are rarely
used due to
photo blurring from lens diffraction, and because these may require prohibitively
long exposure times. For cases where extreme depth of field is desired,
then smaller minimum aperture (larger maximum f-number) lenses allow for a wider
depth of field.

Finally, some zoom lenses on digital SLR and compact digital cameras often
list a range of maximum aperture, because this may depend on how far one has
zoomed in or out. These aperture ranges therefore refer only to the range
of maximum aperture, not overall range. A range of f/2.0-3.0 would mean
that the maximum available aperture gradually changes from f/2.0 (fully zoomed
out) to f/3.0 (at full zoom). The primary benefit of having a zoom lens
with a constant maximum aperture is that exposure settings are more predictable,
regardless of focal length.

Also note that just because the maximum aperture of a lens may not be used,
this does not necessarily mean that this lens is not necessary. Lenses
typically have fewer aberrations when they perform the exposure stopped down
one or two f-stops from their maximum aperture (such as using a setting
of f/4.0 on a lens with a maximum aperture of f/2.0). This *may* therefore
mean that if one wanted the best quality f/2.8 photograph, a f/2.0 or f/1.4
lens may yield higher quality than a lens with a maximum aperture of f/2.8.

Other considerations include cost, size and weight. Lenses with larger
maximum apertures are typically much heavier, larger and more expensive.
Size/weight may be critical for wildlife, hiking and travel photography because
all of these often utilize heavier lenses, or require carrying equipment for
extended periods of time.